The increasing demands for transmitting massive data requires ever increasing signal transmission speeds between components on circuit boards. To achieve these speeds, frequency ranges are necessarily increasing from below 1 MHz to, 1 GHz, 10 GHz or even higher. In these higher ranges, the currents flow mostly near the surface of the conductor due to the well-known “skin effect” which is the tendency of high frequency current density to be highest at the surface of a conductor and to decay exponentially towards the center. The skin depth, where approximately 67% of the signal is carried, is inversely proportional to the square root of the frequency. Accordingly, at 1 MHz the skin depth is 65.2 μm, at 1 GHz it is 2.1 μm, while at 10 GHz the skin depth is only 0.7 μm. At the higher frequencies, the surface topography or roughness of the conductor becomes ever more important since a roughness on the order of, or greater than, the skin depth will impact the signal transmission.
Very Low Profile (VLP) copper foil has a very low roughness. This provides very good performance with respect to signal transmission even at high frequencies. However, VLP copper foil has poor adhesion to the resin layer used in the laminated structures of circuit boards. In fact, the balance between surface roughness for good adhesion and low roughness to provide for good signal transmission presents a challenge. A common approach is to intentionally roughen the conductor surface to improve adhesion characteristics. A surface roughness, Rz, on the roughened surface on the order of μm is typical and, while it can enhance adhesion, it will impact transmission in the GHz range. The design for a good copper foil is therefore constrained by the conflicting need for high roughness to ensure enough adhesion, and low roughness to minimize transmission loss.
There therefore remains a need for copper foils with low transmission loss and good adhesion strength for the manufacturing of circuit boards.